The present invention relates to the distribution of television programs, and more specifically to an improved scrambling scheme for preventing the unauthorized reception of television programs that are scrambled by the suppression of synchronization information.
Techniques for scrambling the video portions of television signals are well known. For example, U.S. Pat. No. 3,813,482 to Blonder discloses a system for transmitting television signals wherein the video is scrambled by suppressing vertical or horizontal synchronization pulses to produce a shifting or rolling scrambled picture. In U.S. Pat. No. 4,542,407 to Cooper, et al, apparatus is disclosed for scrambling and descrambling television programs in which the horizontal synchronizing information is suppressed at a cable television headend, and then regenerated by a subscriber's cable television converter. U.S. Pat. Nos. 4,095,258 to Sperber, 4,163,252 to Mistry, et al, and 4,571,615 to Robbins, et al describe apparatus for decoding scrambled television signals.
The economic viability of subscription television programming, including cable television and satellite television services, is dependent on the ability of the transmitter to encode or scramble a television signal so that an unauthorized receiver will not be able to receive a viewable television picture. As indicated in the patents cited above, a common technique that has been employed to scramble video signals is to suppress the horizontal synchronization pulses below the average value of the video level. This causes the television receiver to unsuccessfully attempt to lock horizontally on random video peaks rather than on the horizontal synchronization pulses. The loss of effective horizontal synchronization prevents the receiver from properly utilizing the color burst signal associated with the horizontal synchronization pulse, so that color reproduction is also faulty.
In order for a receiver to be able to view the scrambled video signal, the suppressed synchronization pulses must be restored. Two techniques are commonly employed to allow the receiver to recover the suppressed sync and timing information. In one technique, a timing pulse is amplitude modulated on the FM audio carrier of the television signal, which is then detected in the audio portion of the receiver and used to generate the timing signals necessary to descramble the received video signal In another known technique, some portion of the sync timing pulses, such as during the vertical interval, is transmitted without suppression, i.e., "in the clear". The receiver phase locks to the clear or unsuppressed sync portion to create the required synchronizing and timing information for descrambling the video portions of the signal.
Neither of these known techniques are immune to unauthorized recovery by "pirate" descramblers. Such descramblers are known that will restore usable synchronization to virtually all known prior art sync suppressed video signals. Such devices typically use the color burst component of the video signal as a "key" to recover all other necessary sync and timing information.
All NTSC (National Television Systems Committee) video signals contain a color burst component at 3.58 MHz. The horizontal frequency of the video signal is related to the burst by a factor of 227.5. Therefore, by extracting the 3.58 MHz color burst components from a received scrambled video signal, a horizontal synchronization pulse can be generated and inserted into the video signal where the original pulse has been removed or suppressed. A television receiver can then lock on to the new sync pulse so that video information can be viewed.
It would be advantageous to provide a system for scrambling the video portion of television signals that would preclude the use of color subcarrier information for regenerating synchronization and timing information. Such a system should not interfere with the descrambling of scrambled signals by the millions of authorized descramblers already installed in the field. The present invention provides such a system.